Investigation of Influencing Parameters for Tribological Conditions in Dry Forming Processes

Environmental awareness and a growing demand for efficient resource utilization encourage the realization of lubricant-free forming processes. A first step in accomplishing dry sheet metal forming is to gain knowledge about the changing tribological conditions and to identify the relevant influencing parameters. The commonly used flat strip drawing test was selected to investigate the tribological conditions in the flange area of deep drawing processes. The influencing factors of contact pressure and varying drawing velocities were analyzed under dry and lubricated conditions. Additionally,the tool and workpiece surfaces were characterized. Besides lubrication, the contact pressure mainly determines the tribological conditions. In lubricated tests higher normal pressure reduces friction, whereas without lubrication higher pressure results in slightly increasing friction. A changing drawing velocity affects the friction when lubricant is applied. In dry experiments, no influence of velocity was found. Results of surface characterization reveal adhesion as main wear mechanism under dry conditions. Based on the investigated influence of the process parameters, an increase in process understanding for dry forming operations is derived.     

圆凹坑织构对线接触摩擦副摩擦学性能的影响

根据摩擦试验中圆盘试样的旋转方向,用YLP-20型激光加工系统在圆柱销试样回转面不同区域(前端、后端、中间)加工规则分布的表面织构,利用UMT-3摩擦磨损仪进行单向旋转摩擦试验,研究表面织构分布区域对摩擦副摩擦学行为的影响。结果表明:在载荷10N、滑动速度0.003~0.628m/s时,不同分布区域的织构对摩擦副的摩擦学行为影响不同,相比无织构试样,分布于试样回转面中间部分的织构对摩擦副起到了减摩作用。这是由于中间织构通过形成局部流体动压润滑作用提高了摩擦副的承载能力,降低了接触表面的摩擦因数,同时通过储存磨屑,减少了表面磨损。前后端织构产生的流体动压润滑效应很小,磨损严重,导致其摩擦因数高于中间织构和无织构试样的摩擦因数。     

Practical determination of friction coefficient of Al 3003 for forming of backward extruded part using simple tip test and inverse finite element analysis

The friction coefficient for aluminum alloy 3003 was determined from a specially designed tip test and finite element (FE) simulations. Measured radial tip distance after the tip test was compared to the FE simulations by iteratively changing friction coefficient and the best fitting friction coefficient was determined. To consider strain rate effect on flow stress response during large plastic deformation, a new combined Hollomon- Voce hardening law was proposed. The friction under three different surface conditions was considered by the proposed inverse FE analysis. The results showed that there was obvious strain rate effect on the predicted punch load in the tip test. Moreover, the different friction coefficients were numerically determined for punch/workpiece and die/workpiece interfaces. Two possible causes of this difference were discussed by the analysis on contact normal pressure and slip velocity distributions of the two interfaces.     

高副接触下沟槽形织构对40Cr表面摩擦性能的影响

目前，面面低副接触情况下织构的减摩降磨性能已经得到广泛的研究，然而针对点面高副接触下的织构对表面摩擦学性能影响的研究仍然较少。主要利用有限元仿真技术建立点面接触下仿生沟槽织构表面流体动压润滑仿真模型，通过ANSYS的Fluent模块进行求解，获取试样表面的润滑油膜承载力与织构几何参数的变化关系。用激光在40Cr试样上加工出仿生沟槽形织构，并采用销盘摩擦副，开展点面高副接触下的织构减摩性能实验研究，综合分析织构几何参数对表面摩擦性能的影响规律。结果表明：试验与仿真具有较高的一致性，随着沟槽织构宽度W和织构深度H的增加，摩擦因数呈先减小后增加的趋势；沟槽织构对改善工件表面摩擦性能具有重要影响，微织构的存在有助于实现流体动压润滑，提高表面承载力，降低摩擦因数，从而改善工件的摩擦性能。     

通过圆弧模具库仑摩擦下的平面拉拔数值滑移线场求解

对通过圆弧模具小变形率库仑摩擦下的平面拉拔问题 ,假设接触面上的压应力分布采用优化方法 ,并结合滑移线场与速度场的数值解法 ,求得该问题的滑移线场、速度场与拉拔力 ,并画出速端图。结果表明 ,该方法不仅可以求解线性indirecttype问题 ,也可以求解非线性indirecttype问题。     

Development of a 45° tilted on-machine measuring system for small optical parts

A new contact type of on-machine measuring system is developed in order to measure aspherical optical parts with steep surface angle for large numerical aperture (NA). A ceramic air slider made of SIALON is adopted for the measurement probe because of its low thermal expansion coefficient, high rigidity and light weight. A high accuracy glass scale is employed to reduce the thermal drift of the displacement gauge. The air slider or the measuring probe is tilted for 45° against the aspherical workpiece axis, and the probe is scanned over the workpiece surface, so as to keep the contact angle between the probe axis and the contact surface constant in order to reduce the change in the probe friction force.     

采油液力马达转子表面织构参数对其摩擦副摩擦学性能的影响

目的研究微沟槽织构对采油液力马达定转子配副表面摩擦学性能的影响,为减小液力马达螺旋副的摩擦阻力矩,从而解决大庆油田某型号液力驱动螺杆泵采油系统停机后启动困难的问题提供设计方向。方法根据螺杆马达螺旋副,建立金属-橡胶平板往复摩擦模型,在金属试件表面加工出不同织构角度和深度的矩形微沟槽,采用正交试验方法,研究不同织构参数对液力马达螺旋副摩擦性能的影响规律,最后再对比分析橡胶试件摩擦磨损试验前后的表面形貌,以掌握织构存在对橡胶定子使用寿命的影响规律。结果织构角度一定时,除90-5号试件以外,其余各组试件的摩擦因数表现出随织构深度的增加而增加的现象。织构深度一定时,各组试件的摩擦因数随织构角度的增大而表现出先增大后减小的现象。织构角度是摩擦因数的主要影响因素。相同试验条件下,0—5号试件的摩擦因数比未织构试件降低了20.2%。结论在液力马达定转子这种金属-橡胶接触对中,织构的减摩机理主要是通过微沟槽输送润滑介质,改善润滑条件。织构参数设计合理的试样,在不缩减液力马达使用寿命的条件下可以有效减小摩擦副的摩擦因数,有利于液力驱动螺杆泵采油系统顺利启动。     

基于C17200与34CrNiMo6材料的摩擦磨损特性与数值模拟研究

目的 探究干摩擦下载荷与速度对于C17200与34CrNiMo6材料摩擦学特性的影响,以探寻C17200材料作为风力机制动闸片的可行性,开展考虑表面粗糙度和接触压力分布不均因素的磨损深度的数值模拟。方法 以C17200与34CrNiMo6材料组成销–盘摩擦副,基于风力发电机的制动工况,利用试验探究其摩擦学特性与磨损机理。在ABAQUS中建立三维销–盘平面/平面磨损模型,设置不同载荷与速度,基于销–盘摩擦副理论模型与UMESHMOTION子程序,结合ALE自适应网格技术,对不同工况下的表面磨损深度进行数值计算,通过试验验证提出的理论模型的合理性。结果 在载荷为3 MPa时,随速度的增加,平均摩擦系数先减小、后增加,速度为125.664 mm/s时,平均摩擦系数取最小值0.575;在速度为62.832 mm/s时,随载荷的增大,平均摩擦系数近似线性增大。载荷为1.5MPa时,平均摩擦系数取最小值0.509。C17200与34CrNiMo6试样的磨损量随速度与载荷的增大而增大,但转速对于磨损量的影响更大。C17200与34CrNiMo6的磨损机理主要为粘着磨损和磨粒磨损。C17200材料磨损...     

A comparison of two models to predict grinding forces from wheel surface topography

Two computer models developed at Trinity College, which can be used to predict the forces generated in grinding and to calculate theoretical grinding charts, are presented. Comparisons are made between the predicted forces from each model and the measured forces from grinding experiments. The first model is based on Challen and Oxley's two-dimensional (2-D) plane-strain slip-line field theory [1] [An explanation of the different regimes of friction and wear using asperity deformation models, Wear 53 (1978) 229–243]. The second model is based on Williams and Xie's three-dimensional (3-D), pyramid-shaped asperity model [2, 3 and 4] [The generation of wear surfaces by the interaction of parallel grooves, Wear 155 (1992) 363–379; The generation of worn surfaces by the repeated interaction of parallel grooves, Wear 164 (1993) 864–872; The prediction of friction and wear when a soft surface slides against a harder rough surface, Wear 196 (1996) 21–34] which generates a series of grooves in the material. Both methods model the grit–workpiece interface as a rigid plastic contact whose behaviour depends on the local grit slope and interfacial coefficient of friction between the asperity and the worn material. Inputs to both models are the material properties of the workpiece. The interfacial coefficient of friction and a calculated distribution of average asperity slopes as a function of increasing depth into the wheel, determined from measured wheel profiles. In addition, the 3-D model uses the calculated asperity density distribution and the distance between adjacent grooves. Grinding tests were performed on bearing steel specimens and tool steel specimens and the forces were measured. The correlation between the theoretical forces from both models and the experimental forces from grinding tests was satisfactory. The most accurate predictions were achieved with the 3-D model.     

Evaluation of stamping lubricants in forming advanced high strength steels (AHSS) using deep drawing and ironing tests

In forming AHSS, the lubricant must reduce the friction between die and sheet as well as the effect of heat generated from deformation and friction, especially in forming at high stroking rates. In this study, the effectiveness of stamping lubricants was evaluated by using the deep drawing and ironing tests. Various stamping lubricants were tested in forming of DP590 GA round cup samples. In these tests, the performance of lubricants was ranked via evaluation criteria that include punch force and the geometry of tested specimens. Deep drawing tests were conducted at two different blank holder forces, BHF (30 and 70 ton) at a constant ram speed (70 mm/s). The ironing tests were conducted to evaluate the performance of lubricants at higher tool–workpiece interface pressure than that is present in deep drawing. Polymer-based thin film lubricants with pressure additives (e.g. Lubricants A and B) were more effective than other lubricants as shown by the force (e.g. maximum punch force and applicable BHF without cup fracture) and geometry indicators (e.g. draw-in length, flange perimeter and sidewall thinning).The pressure and temperature distributions at the die–sheet interface were predicted by FE simulation of deep drawing and ironing tests. As expected, the value of interface pressure and temperature were maximum at the die corner radius.     

Development of tribo-simulator for hot stamping

Coefficients of friction were measured at elevated temperatures using a hot flat strip drawing test machine newly developed by the authors for the purpose of confirming the coefficients of friction used for the FEM simulation of hot stamping. To examine the functions of the tribo-simulator, the coefficients of friction are measured using hot rolling oil with an emulsion, which is supplied continuously, while varying the drawing speed, drawing pressure and temperature. From the experimental results, it was shown that the coefficient of friction in hot stamping can be measured using this tribo-simulator. The measured coefficients of friction can be used as values in the FEM simulation of hot stamping. Moreover, the tribological behavior at the interface between the die and strip in hot stamping can be evaluated from the coefficient of friction, because it is one of the quantitative values used to represent the tribological behavior between the die and blank.     

Friction analysis of thermally sprayed coatings finished by ball burnishing and grinding

Thermally sprayed coatings offer a promising approach as efficient method to increase the wear-resistance of sheet metal forming tools. However, the roughness of thermally sprayed surfaces is quite high. The use of these coatings for deep drawing tools results in poor sheet surface qualities and low drawing ratios. Because it is suspected that high friction is the reason for the low drawability, hard metal coatings (WC–12Co), deposited by high velocity oxygen fuel flame-spraying, were machined by grinding and ball burnishing to improve their friction behavior and the accuracy of the tool shape. The investigation was conducted by plane strip drawing tests. Strips of high strength steel were mated with these novel and effective coatings at different normal contact pressures and drawing velocities. Uncoated friction elements made of C60 steel were considered as reference during the analysis. The results revealed that coated but unmachined friction elements showed high friction values, which led to scratch marks on the sheet surface after drawing. Applying the finishing processes, the friction coefficient could be reduced significantly. Additionally, deep drawing tests were carried out to determine the drawing ratio for coated, unmachined as well as for processed, coated dies. Thermally sprayed and ball burnished as well as thermally sprayed and ground coatings are feasible for deep drawing. Due to the post treatment, the drawing ratio β = 1.8 was increased to 2.0. This is consistent to the results of the friction tests.     

Study on Hot Ring Compression Test of Nimonic 115 Superalloy Using Experimental Observations and 3D FEM Simulation

In hot forging of Nimonic 115, it is desirable to determine friction coefficients. Changing magnitudes of temperature and type of lubricant at the surface of the workpiece and dies influence friction coefficient. This paper describes an experimental investigation of friction under hot forging conditions using the ring compression test. The 3D FEM simulations were used to derive the friction calibration curves and to evaluate material deformation, geometric changes, and load-displacement results. A series of ring compression tests were carried out to obtain friction coefficients for a number of lubricants including mica plate, glass powder, graphite powder, and dry condition. The experiments show how the variations in temperature at the interface affected frictional behavior. On the basis of these results, mica is recommended for hot forging of Nimonic 115 and its friction coefficient is approximately 0.3.     

硅烷改性椰壳粉对热塑性弹性体表面性能的影响及应用

目的研究经硅烷(GPTMS)改性后的椰壳粉(G-CSP)作为添加剂对热塑性弹性体(TPE)表面性能的影响及其潜在应用。方法通过热压法制备G-CSP质量分数为0%、5%、10%和15%的G-CSP-TPE样品,采用Alpha红外光谱仪对GPTMS、CSP和G-CSP进行红外光谱试验。用MFP-D三维形貌仪观测G-CSP-TPE样品的表面形貌,在JC2000CS接触角测量仪上测试不同液体在G-CSP-TPE表面的静态接触角。基于陶瓷球与G-CSP-TPE组成的摩擦副,在MFT-5000摩擦磨损试验机上研究不同载荷和滑动速度下液体介质的摩擦性能。结果红外光谱显示,GPTMS中的某些官能团成功地接枝到CSP中。随着G-CSP质量分数的增加,试样表面粗糙度和接触角均呈增加趋势。去离子水和葵花油的平均摩擦系数与滑动速度和载荷有关,当滑动速度超过2 mm/s时,其平均摩擦系数总体上与载荷呈负相关,在0.8 N和4 mm/s的条件下,去离子水和葵花油的平均摩擦系数差值最大。与其他样品相比,含10%G-CSP的G-CSP-TPE样品区别去离子水和葵花油平均摩擦系数的能力更强,基于该样品,脂肪含量为3.6 g和4.4 g的100 mL两种牛奶的平均摩擦系数分别为0.68和0.576。结论经GPTMS改性后的CSP能够改善TPE的表面性能,含10%G-CSP的G-CSP-TPE样品最有可能被应用于食品口感评价和功能食品的研发。     

Finite element analysis of the effect of friction in high pressure torsion

High pressure torsion (HPT) is one of the most important techniques among various methods that create severe plastic deformation in the production of bulk materials with nano/ultrafine grained microstructures. Since the driving force in deforming the workpiece in HPT is surface friction, understanding of the friction effect is critical for successful application of HPT. In this study, the friction effect in HPT was analyzed using the finite element method. The distribution of effective strain on the contact surface of the HPT samples under different friction conditions was investigated. The friction force influenced the effective strain more in the middle and edge regions than in the central region. The condition for the minimum friction factor that could achieve a sticking condition between the surfaces of the dies, and the samples in the middle and edge regions, was investigated. There was a critical friction coefficient in which the effective strain varies sharply with an increasing friction coefficient.     

Investigations of High Temperature Wear Mechanisms for Tool Steel Under Open-Sliding Contact

During hot working processes, working tools are subjected to severe conditions. Wear is one of the major life limiting factors of the hot working tools. The identification and understanding of the wear mechanism are extremely important for solving problems related to the hot working process. The ultimate aim of this paper is to assess some wear mechanisms of the tool steel used in hot drawing. The tribological tests were performed on high temperature pin-on-disc tribometer with an open-sliding contact for a simulation of hot-drawing process with a refreshed contact surface. The pin material was X40 CrMoV5 steel and the disc material was Fe 360B steel. Experiments were carried out for different disc temperatures ranging from room temperature to 800°C, a constant sliding speed of 50 rev/min and a constant normal load of 70 N. The evolution surface damage and oxides tribolayers have been investigated by SEM and EDS. The results have shown that an increase in test temperature facilitates the generation of oxide and assists in the compaction of the debris, thus producing a wear protective layer, and therefore, a reduction in friction coefficient.     

Thermal behavior of aluminum-coated 22MnB5 in hot stamping under dry and lubricated conditions

The hot stamping of aluminum-coated 22MnB5 has usually been conducted under dry condition, for which the forming load is high since the coefficient of friction is over 0.5. In order to decrease the forming load, the authors previously proposed the use of a lubricant to decrease so that the coefficient of friction from over 0.5 to 0.3. However, it is necessary to understand the heat transfer property in hot stamping under lubricated condition. The purpose of this paper is to examine the heat transfer property of aluminum-coated 22MnB5 in hot stamping under dry and lubricated conditions. In this study, the die and specimen temperatures were measured during compression and compression-sliding tests under dry and lubricated conditions using the hot flat drawing test simulator. In the compression test, the die and specimen temperatures measured under dry and lubricated conditions were the same. On the other hand, in the compression-sliding test up to a sliding distance of 70 mm, the die temperature under lubricated condition was lower than that under dry condition, and it was found that the heat transfer under lubricated condition is superior to that under dry condition. Consequently, there is a difference between the specimen temperatures under dry and lubricated conditions. However, from the results of the tensile test, there is no difference between the tensile strengths under dry and lubricated conditions.     

An experimental study on forming characteristics of pre-coated sheet metals

To analyze the forming and friction characteristics of pre-coated metals (PCMs) that are widely used in household appliances, rectangular deep drawing and friction tests were performed. There were four types of die materials (STD11(TiCN), STD11(TD), STD11, AMPCO) used in the deep drawing tests, and the results show that STD11 and AMPCO are better than others in respect to forming load and surface roughness. The friction mechanism, the thickness of materials and the effect of the roller’s diameter on the friction coefficient and surface roughness were examined in the friction test. The PCMs used in the tests had low friction coefficients (μ=0.15–0.20) in non-lubricated condition due to the coating film acting as lubrication. The friction coefficient decreased as the radius of the roller increased and the surface of product became better as the thickness of the material decreased.     

Friction behavior of aluminum-coated 22MnB5 in hot stamping under dry and lubricated conditions

The coefficient of friction of aluminum-coated 22MnB5 in hot stamping under dry condition is measured for various surface roughnesses of the die using a flat drawing test machine developed by the authors. The surfaces of the die after drawing are photographed and then analyzed using EDX in the SEM. On the basis of the experimental results, the friction behavior under dry condition is discussed. Moreover, the coefficient of friction of aluminum-coated 22MnB5 in hot stamping under lubricated condition is measured for various surface roughnesses of the dies and the effect of the lubrication on the coefficient of friction is discussed.     

Exponential evolution law of fretting wear damage in low-friction coatings for aerospace components

Friction coefficient evolution in low-friction coatings subjected to fretting wear was measured and described using a power-law relation for the development of Kachanov-type damage parameter as a function of accumulated reciprocal sliding distance. Fretting wear tests were performed using idealised contact geometry and loading conditions similar to those found in aero-engine components. A conventional dry film lubricant coating and a mechano-chemical coating were considered, together with various surface pre-treatments, such as high intensity shot peening, shot peening and grit blasting. Results show that friction coefficient growth rate with respect to sliding distance is expressed as a power-law function of the friction coefficient itself. Two parameters were identified that determine the friction coefficient evolution: the damage rate constant and the damage exponent. The damage exponent was found to be close to unity, suggesting an exponential relation between damage and accumulated reciprocal sliding distance, and allowing a mechanical interpretation based on the consideration of energy dissipation in a fretting contact.